Build the EM -10/+4 Level Converter

In the audio world, "professional" studio gear generally features line-level audio inputs and outputs on balanced, three-conductor XLR connectors, operating at a signal level of +4 dBu. In contrast, "consumer" gear uses unbalanced, two-conductor RCA or 11/44-inch connectors, and it sends and receives audio signals at -10 dBV. "Semipro" gear (including most synthesizers and low-cost signal processors) adheres to the consumer standard, except that unbalanced, 11/44-inch connectors are more prevalent than RCAs. (For more on the various types of line-level signals, see "Square One: The Wizard of dBs" in the January 1996 EM.)

Mechanically and electrically, the +4 and -10 audio standards bear little resemblance to each other and rarely coexist without a fight. With some creative wiring, you can often get around some of the major gremlins that crop up when you mingle them in your studio, but curing one problem sometimes creates another. These problems can include hum or buzz, RF noise, impedance mismatches, phase cancellation, and overloaded or underdriven inputs. Such glitches often appear at the most inconvenient times, and they can cause you to pull your hair out in bafflement. You end up scrambling for an extra ground wire, a preamp to boost a signal, an attenuator to reduce another, or one more weirdly wired cable that makes little sense electrically, but hey, it clears up the problem-more or less.

Of course, the simple solution is not to mix different audio standards in the first place. But we must live with the facts that we need both types of gear, that interconnection differences exist, and that we need a reliable method of interfacing different types of equipment.

CIRCUIT OPERATION

In order to go back and forth between the two audio standards, you need two separate circuits: one that translates an unbalanced -10 dBV signal into balanced +4 dBu, and one that does the same thing in the opposite direction. For this project, both circuits have been kept quite simple.

FIG.1: The top portion of this schematic converts -10 dBV to +4 dBu, and the bottom portion converts +4 dBu to -10 dBV.
Click on image to enlarge

The top half of the schematic that is shown in Figure 1 is the consumer-to-professional level converter. An unbalanced signal enters the circuit at capacitors C1 and C2 and is passed to the inverting input of op amp U1A through resistor R1, which sets the input impedance of the circuit to 10 kz. U1A amplifies the signal slightly and feeds it to the noninverting input of U2A. The signal that appears at the right side of R5 is a boosted and inverted version of the original input signal.

U2B inverts the signal from U2A again and feeds it to the output through R8. As a result, the output appearing at pin 2 is an amplified, in-phase version of the input. The signal at pin 3 is a phase-inverted version of this output. Together, they make up the balanced signal output.

The bottom half of the circuit is the pro-to-consumer converter. A balanced input signal is fed through an RF filter made up of R9, C3, R10, and C4. The signal then passes through resistors R11 and R13 to the inputs of U1B, which is wired in differential mode. As a result, this op amp passes the difference between the signals that appear at its inputs.

The strength of balanced circuitry is that any induced signal common to both signal wires is canceled out when it passes through the op amp. The engineering term for this is common mode rejection ratio (CMRR), which is a measurement of a differential circuit's ability to reject unwanted common-mode electromagnetic interference (that is, signals that are in phase in both signal wires). This brings us to the importance of using precision resistors in balanced circuitry.

You'll notice that all resistors specified in the circuits are 1 percent tolerance, metal-film type (see the sidebar "Parts List"). If you're used to building projects with lower-tolerance, carbon-film resistors, you might wonder why precision types are necessary. Strictly speaking, they're not, but if you don't use them at least for critical components (R1, R2, R11, R12, R13, and R14), you're going to have level problems and poor CMRR. The nature of balanced circuitry demands the use of tight-tolerance resistors; if you have access to a 411/42-digit ohmmeter, matching the critical ones to within 0.01 percent or better would not be going too far.

Both circuits were designed using the lowest possible number of parts to minimize signal coloration. In the unlikely event that you can't find the necessary parts at your local electronics shop, you can easily get them all by mail order. I've found that Digi-Key usually has everything I need in stock, and it delivers promptly (tel. 800/344-4539 or 218/681-6674; Web www.digi-key.com).

FIG. 2: Scan this full-size PCB layout into your computer, and use a TTS kit to make your own printed circuit board.

BREADBOARD OR CIRCUIT BOARD?

If you prefer, you can breadboard the circuit, but if you're planning to make more than two converter channels, I recommend using a printed circuit board. PCBs are extremely reliable and rugged, and they are the most time-efficient way to go, even in small quantities. In addition, you'd really have to work hard to make wiring errors, and it's much easier to troubleshoot a well-designed circuit board than to find a problem in a rat's nest of wires and parts sticking out at all angles.

If you're still not convinced, let me tell you about a dead-easy method of making PCBs at home. It's called the Toner Transfer System (TTS), which is made by DynaArt (tel. 813/524-1500; e-mail mail@dynaart.com; Web www .dynaart.com). To do this, you'll need a blank circuit board (copper-clad on one side only), the TTS kit, a suitable etchant (ferric chloride or ammonium/sodium persulphate), a clothing iron, and a hobby drill with a tiny drill bit. You'll also need some way of printing the image; you could use a photocopier, but I prefer to print a scanned image of the circuit board on a laser printer.

The procedure is simple: just cut out the actual-size PCB template shown in Figure 2 and scan it into your computer. Using a graphics program, copy and paste the template as many times as you need for the number of circuits you plan to build and line them up squarely. Print the templates onto a sheet of TTS paper using a laser printer. Cut the templates into pairs, side by side, so they measure 4 by 4 inches.

FIG. 3: The parts are mounted on the opposite side of the PCB from the copper traces.
Click on image to enlarge

Clean the copper side of a 4-by-4-inch circuit board with a powder-type sink cleanser, rinse it, and dry it with a clean cloth. Place the template face up on a hard, flat surface that you won't mind scorching, and orient the circuit board, copper side down, on top of the template. Heat the iron to the "cotton" setting, and place it on the back of the circuit board, applying light but steady and evenly distributed pressure for about three minutes. Lift the iron, let the board cool a little, and place it face up in a bath of room-temperature water. Let it sit there until the paper literally slips off on its own; don't "help" it off, or you'll risk damaging the imprint.

Rinse the board in room-temperature water, place it in the etchant, and leave it there until all the unwanted copper is gone. Using acetone or nail-polish remover, remove the printer toner from the copper traces. You're left with a nice pattern of copper lines that look exactly like the template. Drill the board and mount the parts on the opposite side of the board from the copper lines, following the placement diagram (see Fig. 3).

This might sound complicated, and it does take a few tries to learn, but it's easy once you've made a couple of PCBs. If you've ever made circuit boards using the old photographic method, you will especially welcome this new technique. Give it a try. Be very careful with the hot iron and circuit board, though, and use caution with the etchant, which is toxic and very corrosive. If these things make you nervous, get someone with experience to help you.

CONSTRUCTION HINTS

There are a few golden rules regarding the construction of the level converter that will save you a lot of time and frustration when you get to the point of actually turning the thing on. For example, there are many reasons not to build your own power supply these days. As you've probably noticed, I didn't even include a schematic for one in this project. Ready-made, regulated 12-volt bipolar supplies are widely available, which means that you don't have to mess around with potentially lethal line voltages. (I've used up most of my nine lives on this one; don't tempt fate.)

These power supplies can often be found at your local thrift store for a couple bucks. Look for ones that were originally used for old home PCs, modems, and game computers; they not only have the voltages you need for analog projects, but they have a regulated 5-volt supply, as well, which you will need should you branch out into digital projects. Take me seriously on this; once you've salvaged a used commercial power supply, you'll never consider building another one from scratch for small projects.

Be careful to get the capacitor polarities correct, especially C9 and C10, which will turn into firecrackers if you wire them backward. Make sure the op amps are oriented correctly, too.

Never use bargain jacks; get the best ones you can afford. Gold-plated jacks are ideal if you can mate them with gold-plated connectors.

TESTING

This must be one of the all-time easiest projects to test, because there are no adjustments to be made. First, establish a reference point: put a test CD with a 1 kHz, 0 dB test tone into a consumer-grade CD player. Using a recording device with -10 dBV inputs and a high-resolution bar-graph level meter, adjust the CD player's output level and/or the recorder's input level so that the meter reads exactly 0 dB.

Next, connect the line outputs of the CD player to the -10 inputs on the converter, route the converter's +4 outputs directly to its +4 inputs, and connect the converter's -10 outputs into the recorder's inputs. If you read 0 dB on both channels, everything is working as it should. If the levels are too high, too low, not there, or they sound like a broken analog synth, go back and check for connector-wiring errors, poor solder joints, or hairline cracks in the circuit board. Make sure you have properly identified all the resistor values, as well; the color coding on precision resistors can be tricky to decipher.

APPLICATIONS

In my line of work (forensic audio enhancement), you never know the format of a tape that someone is going to submit for examination. Most of the processing and playback equipment I use is -10 dBV unbalanced (cassette, microcassette, or VHS). When someone came to me one day with a Betacam tape and machine, I had a problem: +4 dBu balanced outputs. I had no way to quickly and properly connect this device to my setup. So I built a level converter with two channels each of -10 to +4 and +4 to -10 conversion, and it deals with most of my interconnection problems very nicely.

Of course, many other applications are possible, especially in a music studio. Suppose you have keyboards strewn around your studio, and the low-level, unbalanced cables running from their outputs to your mixer pick up noise. You can build a 2-channel converter for each one and run a high-level, balanced signal to the mixer inputs instead. This will increase noise immunity dramatically. You can use this trick with any -10 dBV equipment that's located any distance from the mixer.

Say you have a bunch of gear, both balanced and unbalanced, mounted in a rack. The patch bay includes modules that handle -10 and +4 signals, which means you're constantly faced with level problems. Building two channels of two-way level conversion and connecting them to the patch bay will let you deal with many of the problems you encounter on a regular basis.

For the ultimate luxury, build enough converters for all of your semipro and consumer equipment and make all the signals coming to the patch bay balanced. Only one standard signal level and format will appear at all the jacks of the patch bay, and many of your interconnection hassles will disappear entirely. In most personal studios, however, it is more practical to convert a few pro-standard items to the consumer standard. Although this is not as elegant as running +4 dBu balanced lines exclusively, at least everything will be interfaced properly.

If you've ever run into any of the headaches I've mentioned, you owe it to yourself to take the time to build a level converter. Once you've discovered how many problems it can solve, you'll wonder how you ever got along without it. They're so cheap and easy to build, you'll be heating up your soldering iron to make more of these useful gizmos before you know it.

Forensic work can be downright weird at times. To unwind, Peter Mosher likes to create his own "individualistic" electronic music using homemade analog synthesis equipment.